Actinide elements such as americium (Am), curium (Cm) and transcurium elements (berkelium (Bk), californium (Cf), einsteinium (Es) and the like) are α nuclides contained in spent nuclear fuels in the nuclear power industry. When performing reprocessing or partitioning and transmutation of spent nuclear fuels or treatment/disposal of radioactive wastes, it is needed to separate actinide elements from one another, because their nuclear characteristics, exothermic characteristics and radioactive toxicities are different.
With respect to transcurium elements, Cf-252, for example, finds its way into medical applications such as cancer treatment and an application as neutron source for high-level analysis; and is currently produced via neutron irradiation of plutonium (Pu). However, the production thereof is as little as several grams or less per year. Incidentally, one ton of spent nuclear fuels (burnup 150000 MWD/t, cooled for 4 years) generated from a standard fast breeder reactor contains 3 kg of americium, 0.9 kg of curium, 70 μg of berkelium (mainly Bk-249), 3 mg of californium (isotopic mixture of Cf-249, 250, 251, etc.) and 6×10−5 ng of einsteinium (mainly Es-254) and all of them migrate into high-level liquid wastes in the course of the reprocessing.
At present, there are developed a variety of separation, recovery and purification techniques; however, it is very difficult to separate trivalent actinide elements which have similar chemical properties, from one another, and there have been established no effective separation, recovery and purification techniques which offer a prospect of bringing into practice at engineering level.
As methods for separation and purification of americium, which is an actinide element, there have been proposed a solvent extraction method, a solid adsorption method and a precipitation method (refer to, for example, Japanese Patent Laid-Open Specification No. 2000-211925 and Japanese Patent Laid-Open Specification No. 2001-305282). However, all these conventional methods require oxidation operation of trivalent americium to pentavalent or hexavalent americium, and moreover, americium in the highly oxidized state is very unstable, and therefore addition of a chemical reagent is indispensable for its oxidation and maintaining it oxidized state. This makes the system and apparatus used for its separation process complicated, and besides, makes it inevitable to generate secondary radioactive wastes.